Hello vivi,
these pimples and bumps can very easily be linked to your pregnancy, for example, hormones that are secreted during last months of pregnancy can have an effect on small glands in your vaginal area. However, to be on the safe side - which is simply the wisest thing to do in this case, check with your ob-gyn what has caused these bumps and pimples to appear.
Like I said, chances are they're more or less normal part of the pregnancy, but they also might be a sign of an infection that your ob-gyn needs to know about,
Wish you all the best,
Nicole
Vivi,
are they itchy? Does it look like they have puss inside? Are they clustered? You say they are a bit painful - Sometimes herpes can be a bit painful.
If so, this could be herpes - in this case - you do need to tell your obgyn about it. Seriously. If baby is delivered while mother has herpes outbreak it can cause neonatal herpes, which can be fatal for a baby whose immune system isn't strong.
I don't wan't to scare you since herpes is defenitely not the only skin condition that causes "pimples", but since you are in 3rd trimester, it is important to rule out herpes.